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Sept. 15, 2011 -- When it comes to parents thinking about their own teens, they often put blinders on. A national poll finds that parents underestimate the likelihood that their own teenager has used alcohol or marijuana, while overestimating the drug use of other teens.

One in 10 parents surveyed believed their own teenage child had used alcohol during the previous year. And just one in 20 believed their teen had smoked marijuana.

Teens themselves reported a much higher rate of substance use in a separate poll released late last year. About half of 10th-graders said they had used alcohol over the previous 12 months; about one in 4 (28%) reported marijuana use.

More than half of the parents polled in the latest survey believed general alcohol use among teens -- other than their children -- was higher than reflected in the earlier poll and about a third overestimated marijuana use among other teens.

Parents in Denial About Drug Use?

The seeming disconnect suggests a need for better communication between parents and their teenaged children about drug use, but it doesn't necessarily mean parents are in denial or oblivious to abuse issues, says study researcher Bernard Biermann, MD, PhD, of the University of Michigan's C.S. Mott Children's Hospital.

"When teens report that they have used substances, it could mean having a beer at a party six months ago," he tells WebMD. "We really can't say to what extent parents are missing chronic substance use."

The survey included a nationally representative group of parents with children between the ages of 13 and 17.

Among the major findings:

10% of the parents believed their teen had used alcohol within the past year; 15% reported that they were not sure about their child's alcohol use.

Parents of older teens were far more likely to report that their child might be using alcohol or marijuana. Among the parents of 17-year-olds, 23% believed their child had used alcohol and 10% believed their child had used marijuana. Just 3% of parents with 13- and 14-year-olds believed their child might be using the substances.

About a third of parents surveyed estimated the percentage of 10th-graders who used alcohol to be 60% or greater, while about half believed that marijuana use among the nation's 10th-graders was 40% or higher.

Best Defense: Talk to Your Teen

Biermann says parents should never assume that their teens are not vulnerable to drugs and alcohol, adding that parents need to stay vigilant and keep the lines of communication open.

"If your attitude is 'not my child', you will be missing an important opportunity to have the discussion or intervene," he says.

Tom Hedrick is senior communications officer and a founding member of Partnership @ Drugfree.org (formerly Partnership for a Drug-Free America), which conducts its own annual survey to track teen drug use and parental awareness.

"We routinely find that there is a huge gap between the percentage of kids who report experimenting with drugs, alcohol, and tobacco and the percentage of parents who think their child is doing it," Hedrick says.

He says parents should talk to their children about drugs and alcohol even if they don't suspect substance abuse.

"Kids who report learning about the risks of drugs and alcohol from their parents are half as likely to try them," he tells WebMD. "Yet only about 1 in 3 kids report talking to their parents about this."

Lloyd Johnston, PhD, a research scientist at the University of Michigan says it is also important for parents to educate themselves about the drugs being abused in their community.

He cites the rise of the synthetic stimulant known as "bath salts" along with other designer drugs that have become popular with teen drug abusers.

"Things change and parents need to understand that there are always new things coming along," he says.